In the conventional image scanners or facsimiles, the compressions of the black and white binary image data are carried out primarily by softwares, and however, according to this method, the processing time is considerably long, with the result that there is encountered a difficulty in compressing the data with real time. Particularly, in the case where the image data should be inputted at a high speed as in an image scanner, an exclusive compressing apparatus is necessarily required.
As an image data compressing/coding apparatus which complies with the above described requirement, there is provided an image signal coding apparatus which is disclosed in U.S. Pat. No. 4,799,110 patented to Hisada et al. This conventional image signal coding apparatus comprises reference and coding line pixel change point detecting circuits, reference line shift registers, a coding line shift register, a symbol detecting circuit, coding and reference line ROM tables, and a code determinating circuit.
This conventional apparatus selectively uses MH(Modified Huffman) coding scheme known as a one dimensional coding scheme, MR(Modified READ) coding scheme or MMR(Modified Modified READ) coding scheme, the latters being known as two dimensional coding scheme, thereby making it possible to selectively code one dimensional image data or two dimensional image data.
Thus, due to the fact that the above conventional apparatus is capable of using plural coding schemes, separate processing means for the reference line has to be provided in addition to processing means for coding line, so that the construction of the apparatus become very complicated. Further, during the final process of generating the codes for the coding line image signals, the ROM table receives the output of the symbol detecting circuit in addition to the output of the run length couter, and further cooperates with the code determinating circuit, thereby making the sequence of the processing complicated.
Meanwhile, on considering the characteristics of the black and white binary image data to be coded, it can be seen that there are high probabilities of the identical color pixel occurrence in successive pixels. That is, if a black pixel appears, then the pixels of the identical color continuously appear for a certain length. Successively, a white pixel appears, then the pixels of the identical color continuously appear. In such a succeeding pixels, a pixel of a certain color next to the final pixel of the preceding image data of different color is defined as "a change point", and the number of the succeeding pixels of image data representing an identical color as "a run length" in the art. The usual data compression is carried out based on the generation frequency of such a run length. For example, the shortest codes are allocated to the run length of which the generation frequency is highest, while the longest codes are allocated to the run length of which the generation frequency is lowest. Thus, this coding method is called a variable length coding method, and the adoption of this method is advantageous for enhancing the data compressing efficiency.
For this reason, the one dimensional image data codes have been made to comply with the modified Huffman coding scheme according to the T4 regulation recommended by CCITT (International Consultative Committee for Telephone and Telegraph).